How to deal with Arak is one of several issues that must be tackled in negotiations between Iran and six global powers that got under way in February with the aim of reaching a long-term deal on the decade-old nuclear dispute by late July.

Princeton University academics said that annual production of plutonium could be cut to less than a kilogram – well below the roughly eight kg needed for an atomic bomb – if Iran altered the way the plant is fuelled and lowers its power capacity.

"These redesigns would not reduce the usefulness of the reactor for making radioisotopes and conducting research," wrote Ali Ahmad, Frank von Hippel, Alexander Glaser and Zia Mian – members of Princeton's Program on Science and Global Security.

"This approach would meet Iran's needs and would address the concerns of the international community," said their article, due to be published on Wednesday by the online journal of the Arms Control Association, a US research and advocacy group.

Iran denies Western allegations that it is seeking the capability to make nuclear bombs, saying its program is aimed at generating electricity and carrying out peaceful research.

Experts from Iran and the United States, Russia, France, Germany, Britain and China are due to meet in Vienna for three days from Thursday to be followed by a third round of political-level talks next week.

Western powers fear Arak could provide a supply of plutonium – one of two materials, along with highly enriched uranium, that can trigger a nuclear explosion – once operational.

The Islamic Republic has said that the 40-megawatt, heavy-water reactor is intended to produce isotopes for cancer and other medical treatments. Iran agreed to halt installation work at Arak under an interim deal reached with the powers last year.

Their positions seem far apart. Iran has ruled out shutting down any nuclear site, including Arak, which has been under construction for years. The United States says it sees no need for Arak as part of a civilian nuclear program.

'Win-win solution for Arak'

However, the head of Iran's atomic energy organization, Ali Akbar Salehi, in February signaled some flexibility, saying it was prepared to modify Arak to help allay any concerns.

Heavy-water reactors, fuelled by natural uranium, are seen as especially suitable for yielding plutonium. To do so, however, a nuclear reprocessing plant would also be needed to extract the plutonium. Iran is not known to have any such plant.

If operating optimally, Arak could produce about nine kg of plutonium annually, the US Institute for Science and International Security says.

Any long-term deal must lower that amount, experts say.

The Princeton University experts said that changing Arak's fuelling and operating power would make it less of a proliferation concern, even if it were to remain a heavy water-moderated reactor.

"The conversion steps described above are technically feasible," they said in the article titled "A Win-Win Solution for Iran's Arak Reactor".

Robert Einhorn, a former US State Department official on Iran, said that at a minimum using enriched uranium fuel and reducing the power level would be required for Arak. But he said it would preferably also be converted to a light-water reactor, a more extensive reconfiguration step that Iran may resist.

Arak was still a less immediate concern than Iran's existing uranium enrichment program, which gives it the ability to produce enough weapons-grade uranium for a bomb in a couple of months, Einhorn said in a new report.